Pat the meat dry, sprinkle with kosher salt and freshly ground pepper and dredge in flour.
Heat 2 tablespoons oil in large Dutch oven over medium-high heat. Once hot and ripping, add half the meat and brown, and then remove to plate. Repeat with the second batch of meat. Once all the meat is evenly browned, reduce the heat a bit and add another drizzle of oil if necessary. Then add the onions, garlic and bay and cook to soften, 7 to 8 minutes. Add the tomato paste and stir a minute. Then stir in the beer and reduce by half, a minute or 2 more. Add the Worcestershire, stock, horseradish and beef. Add just enough water to surround the meat and partially cover. Simmer over medium-low heat for 2 hours, until very tender. Cool and store in the fridge for a make-ahead meal.
To serve, quarter the potatoes, cover with water in a medium pot and bring to boil. Salt the water and cook to just-tender, 10 to 12 minutes, tops. Drain and return to the hot pot. Add the butter and parsley and stir to melt the butter.
Place a small pile of buttered potatoes in each shallow bowl, top with stew and garnish with watercress leaves. Serve with pumpernickel or sourdough bread.
Cook's Note: When storing bread in your kitchen for a few days, wrap tightly in plastic to keep tender, and crisp in a hot oven. Do not refrigerate.